In my prior U.S. Pat. No. 4,246,784, I have disclosed a technique for the passive non-invasive temperature measurement of the interior of a body using the acoustic thermal noise spectra of the body. According to that technique, one or more acoustic transducers are coupled to the surface of the body to intercept the acoustical noise signal from within the interior of the body along well defined paths to generate a corresponding electrical signal. The noise power spectrum of the electrical signal is analyzed by means of a power spectrum analyzer to develop an output representing the temperature-depth distribution along said paths.
The subject invention is directed to a new technique of radiation-induced thermoacoustic imaging for obtaining information non-invasively on the composition and structure of a material or body and has particular application to the imaging of soft tissue in humans and animals as well as other moderately homogeneous materials and bodies. Recent progress by the inventor and others in understanding thermoacoustic detection of charged particles shows that (a) there is a simple and direct connection between the pattern of induced thermal stress and the received pressure signals, and that (b) the signal-to-noise ratio of thermoacoustic signals relative to thermal noise can be reliably calculated. The results, when applied to thermal stress pulses induced in soft tissues by therapeutic ionizing radiation or non-ionizing (radio-frequency (RF), microwave, ultrasonic) radiation, indicate the feasibility of thermoacoustic imaging. Such images would permit (a) verification of treatment plans with respect to the positioning of therapeutic radiation dosage and (b) non-invasive identification of tissue characteristics which are not sensed by other imaging modalities.
It is known from the study of thermoelasticity by engineers, of auditory sensing of microwave pulses by biomedical researchers, and of acoustic detection of charged particles by physicists that the sudden thermal expansions due to heat pulses result in the emission of acoustic words. F. Braun, in Ann. d. Physik 65, 358 (1898), reported that temperature variations in a gas produced by passage of current through a fine resistance wire provide a source of acoustic waves. This effect was later employed in a precision sound source called the thermophone. Fairbank et al reported success with the thermophone as a sound source in liquid helium and liquid hydrogen in an article entitled "The Thermophone as a Source of Sound on Liquid Helium and Liquid Hydrogen," J. Acoust. Sec. Am. 19, 475 (1947).
In mechanical engineering the study of elastic deformation of bodies has evolved for more than a century, but the field of thermoelasticity, the study of elastic deformation due to thermal stresses, has mostly developed since World War II. Several textbooks are now available which discuss a wide range of problems in thermoelasticity. One class of solutions relates to the dynamic problem: where the thermal stress is applied suddenly, so the transit time for the propagation of elastic stresses (at the speed of sound) cannot be neglected. If the diffusion of heat as time progresses is properly taken into account, the problem is generally too complicated to be solved in closed form with simple analytic expressions. Fortunately, for considerations of thermoacoustic imaging, it is an excellent approximation to neglect heat diffusion. If .DELTA.x is the size of the smallest region under consideration (.DELTA.x.gtorsim.10.sup.-2 cm for soft tissue imaging because of the strong high frequency attenuation of ultrasound), c is the speed of sound (c.congruent.1.5.times.10.sup.5 cm/s), and D is the thermal diffusion coefficient (D.about.10.sup.-3 cm.sup.2 /s), then heat diffusion may be neglected if c.DELTA.x/D&gt;&gt;1; for soft tissue, c.DELTA.x/D.about.1.5.times.10.sup.6. When heat diffusion is neglected, the solution, as will be discussed later, is fairly simple, especially if one thinks in terms of retarded potentials in analogy to those in electromagnetic theory.
Among biomedical researchers there has been considerable work in recent years which has established that the auditory sensing of microwave pulses can be explained by thermoacoustic waves induced by the sudden thermal stress due to absorption of energy from the microwave radiation field. The threshold for an auditory response in humans is most simply related to the energy deposition per unit mass (or volume) per pulse. For humans, this threshold is typically .about.16 mJ/kg and for cats .about.10 mJ/kg. Assuming a tissue density .rho..apprxeq.1, the former figure is equivalent to 160 ergs/cm.sup.3 -pulse=1.6 rad/pulse. (The rad is a very convenient unit for RF or microwave heating dose, especially since we wish to make comparisons with thermoacoustic emission induced by therapeutic ionizing radiation.) Since the microwave pulse lengths are typically 1-10 .mu.sec, the acoustic frequencies extend up to .apprxeq.0.1-1 MHz. Therefore, the ear responds to only the small fraction of the emitted acoustic energy spectrum which falls in the audio frequency range. A wide-band acoustic detector optimized for maximum S/N ratio would certainly have a lower dose threshold than the auditory system.
It may be useful to briefly recite the events which led the inventor to thermoacoustic imaging. After participating in an interdisciplinary study group on ultrasonic imaging techniques in medicine, the inventor remarked in August 1975 to a group of cosmic ray scientists that the possibility of acoustic detection should be considered in connection with a proposed Deep Underwater Muon And Neutrino Detector (DUMAND) as an adjunct to the detection of the very feeble light emission by neutrino interactions in deep ocean water. A year later, the inventor and a Russian physicist independently presented calculations to the 1976 DUMAND Workshop which indicated that acoustic detection of high energy particle events might be feasible. See T. Bowen, "Sonic Particle Detection," Proceedings of the 1976 DUMAND Summer Workshop, University of Hawaii, Sept. 6-19, 1976, p. 523, and B. A. Dalgoshein, ibid., p. 534. The inventor's calculation began with a solution in the time domain (as distinguished from the frequency domain) for an instantaneous heat pulse given by Norwacki in his textbook entitled Thermoelasticity, Addison-Wesley, Reading, MA/Pergamon, Oxford (1962), at page 266. This led to a very simple way of thinking about thermoacoustic phenomena. The favorable predictions caused a number of the workshop participants, including the inventor, to form a collaboration to carry out experimental investigations in high energy proton beams. By the time the first experimental results were obtained in late 1976, a few mistakes in the first theoretical estimates were corrected, and excellent agreement was found between the thermoacoustic theory and experimental results for heating by pulses of ionizing radiation. See L. Sulak et al., "Experimental Studies of the Acoustic Signature of Proton Beams Traversing Fluid Media, " Nucl. Instrum. and Methods 161, 203 (1979).
It was clear from the experimental and theoretical work that the acoustic signal from a single high-energy-neutrino-produced event would be very weak, and might be lost in background noise. This led the inventor at the 1977 DUMAND Workshop to work on the problem of calculating the signal-to-noise (S/N) ratio. At the suggestion of the underwater sound experts who were also participating in the workshop, the inventor studied the general theory of optimal filtering to maximize S/N and to estimate the best possible S/N. The inventor learned that the optimal S/N could be calculated for neutrino detection in a straight-forward manner. See T. Bowen, "Theoretical Prediction of the Acoustic Emission from Particle Cascades, and the Signal-to-Noise Ratio," Proceedings of the La Jolla Workshop on Acoustic Detection of Neutrinos, Scripps Institution of Oceanography, July 25-29, 1977, p. 37. Another participant, J. Learned, later applied this technique to detailed calculations for neutrino events of any energy and spatial orientation as reported in "Acoustic Radiation by Charged Atomic Particles on Liquids, an Analysis," Phys. Rev. D19, 3293 (1979).
Ironically, although the DUMAND collaboration inspired a great deal of theoretical and experimental understanding of low level thermoacoustic pressure signals, it became clear by 1979 that the energy threshold for detecting the most interesting neutrino events is too high. See T. Bowen and J. Learned, "Acoustic Detection of Ultra High Energy Neutrinos," Proceedings of the 16th Int. Cosmic Ray Conf., Kyoto, Japan, August 1979, Vol. 10, p. 386, and T. Bowen, "Acoustic Detection of Ultra High Energy Neutrinos in the Deep Ocean in the Presence of High-Level Low-Frequency Noise," Proceedings of the 1979 DUMAND Symposium and Workshop, Khabarovsk and Lake Baikal, U.S.S.R., August 1979.